The Nereus Hybrid Underwater Robotic Vehicle for
Global Ocean Science Operations to 11,000m Depth
Andrew D. Bowen, Dana R. Yoerger, Chris Taylor, Robert McCabe, Jonathan Howland,
Daniel Gomez-Ibanez, James C. Kinsey, Matthew Heintz, Glenn McDonald, Donald B. Peters1
Barbara Fletcher, Chris Young, James Buescher2
Louis L. Whitcomb, Stephen C. Martin, Sarah E. Webster, Michael V. Jakuba1,3
Abstract— This paper reports an overview of the new Nereus
hybrid underwater vehicle and summarizes the vehicle’s performance during its first sea trials in November 2007. Nereus
is a novel operational underwater vehicle designed to perform
scientific survey and sampling to the full depth of the ocean
of 11,000 meters — almost twice the depth of any present-day
operational vehicle. Nereus operates in two different modes. For
broad area survey, the vehicle can operate untethered as an
autonomous underwater vehicle (AUV) capable of exploring and
mapping the sea floor with sonars and cameras. For close up
imaging and sampling, Nereus can be converted at sea to operate
as a tethered remotely operated vehicle (ROV). This paper
reports the overall vehicle design and design elements including
ceramic pressure housings and flotation spheres; manipulator
and sampling system; light fiber optic tether; lighting and
imaging; power and propulsion; navigation; vehicle dynamics
and control; and acoustic communications.
I. I NTRODUCTION
The goal of the Nereus project is to provide the U.S.
oceanographic community with the first capable and costeffective vehicle for routine access to the world’s oceans to
11,000 meters [6]. This paper reports an overview of the the
new Nereus hybrid underwater vehicle and summarizes its
performance during its first sea trials. Nereus is a novel operational underwater vehicle designed to perform scientific survey
and sampling to the full depth of the ocean of 11,000 meters
— almost twice the depth of any present-day operational
vehicle. Nereus operates in two different modes. For broad area
survey, the vehicle can operate untethered as an autonomous
underwater vehicle (AUV) capable of exploring and mapping
the sea floor with sonars and cameras. For close up imaging
and sampling, Nereus can be converted at sea to become a
tethered remotely operated vehicle. The ROV configuration
incorporates a novel lightweight fiber optic tether to the surface
for high bandwidth real-time video and data telemetry to the
surface, enabling high-quality remote-controlled teleoperation
by a human pilot. Figure 1 depicts the tethered ROV mode
mode concept of operations. Figure 2 shows Nereus in its AUV
mode (left image) and ROV mode (right image).
1 Department of Applied Ocean Physics and Engineering, Woods Hole
Oceanographic Institution, Woods Hole, MA
2 U.S. Navy Space and Naval Warfare Systems Center, San Diego, CA
3 Department of Mechanical Engineering, The Johns Hopkins University,
Baltimore, MD
Fig. 1. Nereus ROV-mode concept of operations. In ROV-mode Nereus
is remotely controlled by a lightweight expendable fiber optic tether which
connects the vehicle to a surface support vessel.
Nereus’s first sea trials were conducted in November 2007
from the R.V. Kilo Moana in the Pacific Ocean near Oahu,
Hawaii. The sea trials demonstrated vehicle operations in AUV
mode and ROV mode to a depth of 2270 meters. Figure 3
summarizes the Nereus sea trial dive statistics. Future sea
trials are planned to demonstrate the vehicle’s operational
capability for operations at depths to 11,000 m and with
extreme horizontal mobility.
The Nereus vehicle project lead institution is the Woods
Hole Oceanographic Institution with collaboration of the Johns
Hopkins University and the U.S. Navy Space and Naval Warfare Systems Center San Diego. The Nereus vehicle project is
supported by principally by the National Science Foundation,
with additional support provided by the U.S. Navy Office
of Naval Research, the National Oceanic and Atmospheric
Administration, the Woods Hole Oceanographic Institution,
978-1-4244-2620-1/08/$25.00 ©2008 IEEE
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Fig. 2. The Nereus hybrid remotely operated vehicle, shown during its first engineering sea trials in the Pacific in November 2007, is designed to operate in
two modes to depths of 11,000m. Left: Nereus configured for autonomous vehicle survey operations. Right: Nereus configured with a light fiber optic tether,
a robot arm, sampling gear, and additional cameras for teleoperation of close-up imaging, sampling, and manipulation missions.
and the Russell Family Foundation.
II. BACKGROUND AND S CIENTIFIC R ATIONALE
Existing deep submergence vehicle systems have excellent
capabilities and provide critical, routine access to the sea floor
to a maximum depth range of 6,500 m — e.g. the 4,500 m
Alvin human occupied submersible [7], [19], the 4,500 m ABE
AUV [35], [36], and the 4,000 m Tiburon ROV [26]. Only a
few presently operational U.S. vehicles are capable of diving to
6,500 m and conducting high resolution mapping and sampling
— e.g. the 6,500 m Jason II ROV [33]. These capabilities have
led to significant scientific discoveries over the past 30 years
including identifying and sampling mid-ocean ridge volcanic
processes, hydrothermal processes, and biological ecosystems
which have revolutionized the biological sciences [1]. Progress
in deep sea research at ocean floor sites between 6,500 m
and 11,000 m has been hindered by a lack of suitable costeffective vehicles that can operate at these depths. Given the
need for full access to the global abyss, and national and
international imperatives regarding ocean exploration, a variety
of studies have identified the development of an 11,000 m deep
submergence vehicle as a national priority [1]–[3], [28]
To date, only two vehicles have ever reached the deepest
place on Earth — Challenger Deep of the Marianas Trench
at 11◦ 22’N, 142◦ 25’E in the Western Pacific Ocean near
the island of Guam [13]. On January 23, 1960 the humanpiloted Bathyscaph Trieste, developed by Auguste Piccard,
made one successful dive to the Challenger Deep [27]. In 1995
the remotely controlled ROV Kaiko, built and operated by
the Japan Agency for Marine-Earth Science and Technology
(JAMSTEC), made the first of several successful dives to the
Challenger Deep [31]. Neither Trieste nor Kaiko is presently
operational. Moreover, the design approaches employed in
these two (very different) vehicles necessarily result in high
operational costs — too costly to be routinely supported by
United States oceanographic science budgets.
The depth capability of conventional tethered ROVs such
as Jason II cannot be directly extended to 11,000 m because
conventional steel-reinforced cables are self-supporting in sea
water only to cable lengths up to about 7,000m. Alternative
tension member materials for 11,000 m operations, e.g. Kevlar,
result in large-diameter cables that exhibit poor hydrodynamic
characteristics and that require very large cable handling
systems
Light fiber optic tethers offer an alternative to conventional
large-diameter steel and Kevlar cables. To date, light fiber
tethers have principally been employed in military applications; relatively few light fiber tether systems have been
employed for oceanographic research. In [5], [23]–[25] the
authors report the development of the self-powered remotely
operated vehicle UROV7K employing a fiber-optic tether. This
vehicle is designed to operate exclusively as a tethered ROV,
and does not have on-board computational resources necessary
to operate autonomously. In [8], [10] International Submarine
Engineering Limited reported the successful deployment of an
autonomous underwater vehicle designed to deploy fiber optic
communication cables on the arctic sea floor.
Our goal is to create a practical 11,000 m system using an
appropriately designed self-powered vehicle that can (a) operate as an untethered autonomous vehicle (AUV mode) and (b)
operate under remote-control connected to the surface vessel
by a lightweight fiber optic tether of up to approximately 40
km in length (ROV mode).
III. H YBRID V EHICLE D ESIGN OVERVIEW
The Nereus core vehicle employs twin free-flooded hulls.
It can be re-configured at sea into ROV-mode or AUV-mode.
All on-board electronics, batteries, and internal sensors are
housed at 1-atmosphere in novel lightweight ceramic/titanium
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Date
Dive
11/20/2007
11/21/2007
11/23/2007
11/23/2007
11/24/2007
11/24/2007
11/25/2007
11/26/2007
Freejoy
NER000
NER001
NER002
NER003
NER004
NER005
NER006
Vehicle
Mode
AUV
AUV
ROV
ROV
ROV
ROV
ROV
AUV
Time
Submerged
5h 56 min
2h 12 min
57 min
5h 56 min
1h 20 min
3h 47 min
6h 50 min
11h 33 min
Fig. 3.
Distance
Covered
3940 m
2319 m
0m
1751 m
0m
2236 m
2270 m
10843 m
Depth
2m
4m
18 m
398 m
100 m
569 m
2257 m
22 m
Time
On
Bottom
0 min
0 min
0 min
4h 55 min
0 min
2h 43 min
3h 21 min
12 min
Fiber Payout
no tether
no tether
28 m
978 m
1368 m
1422 m
2352 m
no tether
Dive Statistics: November 2007 Nereus Sea Trails
pressure housings developed specifically for this project [30].
Additional buoyancy is provided by lightweight hollow ceramic buoyancy spheres [29], [32]. Two 0.355 m OD ceramic
pressure housings contain power switching and distribution
systems, DC-DC power isolation, a Linux control computer,
a Linux imaging computer, DC-brushless motor controllers,
multiple gigabit Ethernet transceivers, strap-down navigation
sensors, and external sensor and actuator interfaces. Nereus’s
power is provided by an 18 kWh rechargeable lithium ion
battery pack, developed for this project, contained in two
ceramic pressure housings. Cameras, emergency beacons, RF
modem (for surface operations), and other electronics are
housed separately in dedicated 0.191m OD ceramic and titanium pressure housings. Lighting is provided by lightweight
ambient-pressure light-emitting diode (LED) arrays custom
developed for the Nereus project [16]. Pressure balanced
oil filled junction boxes and hoses provide vehicle electrical
and optical interconnect. The vehicle navigation sensor suite
is described in Section IX. The vehicle’s large metacentric
height provides passive stability in roll and pitch. Twin aft
vertical stabilizers (i.e. fixed vertical tails) provide passive
hydrodynamic stability in heading.
A. Nereus AUV Mode Configuration Summary
utility cameras, and several LED arrays. ROV mode propulsion
is provided by two 1 kW thrusters fixed on the aft tails, one
lateral 1 kW thruster, and one vertical 1 kW thruster.
Fig. 4.
Main vehicle flotation structure for one hull (one of two).
IV. C ERAMIC H OUSINGS AND B UOYANCY S PHERES
Ceramic buoyancy spheres and custom ceramic-titanium
pressure housings were essential to minimizing the vehicle
overall size and mass. This section reviews their design and
development.
In AUV mode, Figure 2 (left), Nereus is neutrally buoyant
with a displacement of 2,500 kg with 1,510 ceramic buoyancy
spheres and a reserve payload buoyancy of 22 kg. This mode
employs two independently articulated actively controlled foils
(wings) located between the hulls at the aft and middle
sections, respectively. AUV mode propulsion is provided by
two 1 kW thrusters fixed on the aft tails and one 1 kW thruster
on the articulated mid-foil. AUV mode has no lateral thruster
actuation. The vehicle is hydrodynamically stable in pitch and
heading when in forward flight. A downward looking survey
camera and several LED arrays are mounted on the port hull.
B. Nereus ROV Mode Configuration Summary
In ROV mode, Figure 2 (right), Nereus is neutrally buoyant
with a displacement of 2,700 kg with 1,670 ceramic buoyancy
spheres and a reserve payload buoyancy of 45 kg. This mode
adds a work package containing a 6 degree-of-freedom (6DOF) electro-hydraulic robot arm, sampling tools, sample
containers, an additional high-resolution digital camera, two
Fig. 5. A main ceramic pressure housing being readied for fitting into Nereus.
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A. Ceramic Buoyancy Spheres
Ceramic flotation was selected because of its low weight to
displacement characteristics. 99.9% alumina ceramic seamless
spheres, of 91mm OD, manufactured by Deep Sea Power
and Light (DSPL) in San Diego, California, were chosen
[32]. These spheres each weigh 140g and displace 404g in
sea water, for a nominal 0.35 weight-to-displacement ratio.
The spheres supplied by DSPL were individually tested to
30kpsi external pressure. The spheres are individually jacketed
with 5mm thick elastomeric polyvinylchloride (PVC) “boots”
that provide robust protection against impact loading, while
providing an additional 19g of net buoyancy. The booted
spheres each produce 283g of buoyancy at the surface. The
spheres are less compressible than water, so at full operating
depth of 11,000m they each generate 306g of buoyancy.
The main vehicle flotation consists of 1472 spheres arranged
in the upper portions of the vehicle hulls, generating 417kg of
net buoyancy. The spheres are housed in longitudinal tubes
within buoyant polypropylene modules. The polypropylene
modules are constructed as arrays of 25mm plate stock held
together with aluminum tie rods. The plates are oriented
vertically, and have patterns of circular cutouts that form
tubular holes when the modules are assembled. The polypropylene structure supporting the main vehicle flotation has a
total weight of 690kg and displacement of 778kg, for a net
buoyancy of 88kg. Total buoyancy generated by spheres and
polypropylene structural modules, Figure 4, is 505kg at the
surface. Due to the fact that water is more compressible than
both the spheres and the plastic structure, the total buoyancy
at 11,000m depth is 573kg.
B. Ceramic Pressure Housings
Ceramic was selected for pressure housing material because
its high compressive strength-to-weight ratio allows for nearneutrally buoyant housings capable of going to these extreme
depths. Equivalent titanium housings of this size and quantity
would weigh hundreds of kilograms in water and require
expensive and voluminous additional flotation to offset their
weight.
The housings, Figure 5, are composed of 96% alumina ceramic, and were custom manufactured for Nereus by CoorsTek
in Golden, Colorado. Two size housings were developed —
355mm and 191mm OD. The vehicle main electronics and
batteries are contained in four of the 355mm housings. For
cameras and auxiliary electronics, the 191mm housings are
used. The housings consist of a ceramic section formed by
CoorsTek, and titanium joint rings manufactured at WHOI and
bonded to the ceramic with a high-strength epoxy material.
The ceramic pieces are formed in both cylindrical sections
(both ends open) and capped-cylinder sections (one end hemispherical). Due to manufacturing limitations, the length of the
ceramic sections was limited to about 500mm, so for the main
housing design a capped cylinder was joined with an open
cylinder to provide sufficient length. Hemispherical titanium
end caps are used as housing closures, and all necessary
penetrations for electrical and optical conductors, as well as
purge ports, were provided for in these titanium end caps.
Finite element analysis (FEA) was performed to ensure
the matching of deflections of the ceramic and titanium
components under pressure, to ensure even axial loading on
the ceramic components. Strain gaging and acoustic emissions
measurement were performed during pressure testing of all
housings to a proof pressure of 18,000 psi. For additional
information the reader is referred to [30].
V. F IBER O PTIC T ETHER
A key part of the HROV system is the light weight fiber
optic cable used when operating in the ROV mode. Because
HROV is a light weight battery powered system it is limited in
its available energy and weight. The fiber optic tether transmits
high bandwidth data only, not power.
Fiber Parameter
Diameter
Specific Gravity (FW)
Weight of 11 km in water
Working Strength
Breaking Strength
Survivability on Seafloor
Fig. 6.
FOMC
0.8 mm
1.74
4.23 kg
133 N
400 N
Good
Buffered Fiber
0.25 mm
1.36
0.173 kg
8N
108 N
Poor
Candidate Fiber Tether Mechanical Specificatoins
The preliminary design analysis of this cable for deepocean deployments is described in [37]. Two cable designs
were selected as candidates for the HROV system: Fiber
Optic Microcable (FOMC) and the Sanmina/SCI buffered
fiber. Characteristics of these cables are given in Figure 6.
After extensive simulation using the WHOICABLE program,
[14], [15] the Sanmina/SCI tether was selected as the primary
choice for the HROV system.
The basic concept of the deployment system involves the
use of a snag resistant depressor and vehicle package to
house the tether system as shown in Figure 7. The depressor
was designed to get the upper tether deployment point below
surface currents and below the most energetic and biologically
active part of the water column. The vehicle package contains
the optical fiber dispenser, brake, fiber counter and cutter, and
it is designed to minimize drag and the chance of snagging
the fiber. The depressor and vehicle package are mated together during launch, protecting the fiber during the transition
through the air-water interface. Once the system has reached
a designated depth, the vehicle package separates from the
depressor. Fiber optic tether pays out from both the vehicle
and the depressor as the vehicle descends.
The cable deployment system was integrated with the actual
Nereus HROV vehicle in fall 2007, and reported on in further
detail in [11]. Initial sea trials were conducted near Hawaii
in November 2007. Operational procedures for launch and
recovery were developed and tested. Four dives were made
in the ROV configuration using the fiber, culminating in a
ROV mode dive to 2270 meters. During all ROV dives, the
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has a small, lightweight gear pump coupled to a brushless DC
servo motor where speed and torque can be easily controlled.
thereby offering control strategies resulting in minimal power
usage at all times. The maximum power used at any time
by the HPU is limited to 750 watts. The hydraulic fluid is a
biodegradable mineral oil based, ISO 7 marine hydraulic fluid,
developed specially for Nereus’s extreme ambient pressures.
A sampling platform is integral to the tool sled and provides
storage facilities for science equipment and samples. Both
the sampling platform and overall tool sled were designed in
conjunction with the manipulator in an effort to harmonize the
kinematic capabilities of the manipulator system with its work
space. The manipulator is placed on the platform such that it
has a full 270◦ of reach on the starboard corner of Nereus
within the camera views. This provides a useful reach area of
approximately 5.4 sq. m. of sea floor.
Fig. 7.
Nereus showing the float-pack separating from the depressor pack.
VII. C AMERAS AND L IGHTING
fiber remained intact until purposely cut at the completion of
operations.
VI. N EREUS M ANIPULATOR AND S AMPLING S YSTEM
The Nereus sampling system consists of a 2.4 m x 1.2
m platform with a custom designed Kraft TeleRobotics manipulator and a WHOI designed hydraulic power unit. The
manipulator is 7-function, 6 degrees of freedom, closed-loop,
position controlled master slave system. The kinematics were
developed to maximize the Nereus work space, and it has the
following specifications.
Manipulator Parameter
Horizontal reach
Lift capacity at full extension
Controllable grip closure force
Stowed height
Wrist rotate torque
Weight in air (sea water)
Electrical Power Input
Operating depth
Shoulder Azimuth
Elbow Pivot
Wrist Yaw
Shoulder Elevation
Wrist Pitch
Wrist Rotate (slaved mode)
Wrist Rotate (cont. mode)
Jaw Opening (4-finger)
Fig. 8.
Value
60”
30+ lb
0-100 lbf
41.5”
180 in-lb
105 lb (70lb)
100W to 750W (max)
11,000 msw
270◦
120◦
105◦
120◦
154◦
340◦
0-10 rpm
8.75”
Nereus Electro-Hydraulic Manipulator Performance Specifications
The Nereus hydraulic power unit (HPU) provides hydraulic
pressure and flow to the manipulator. This is a WHOI designed
modular hydraulic system that delivers 0-1000 psi at flow rates
of 0-2 gpm. The HPU uses the same DC brushless motor and
controller as employed for the Nereus propulsion system. It
Fig. 9. Nereus’s ambient-pressure LED-arrays provide both strobed and
continuous illumination.
The power limited nature of Nereus required careful consideration of the requirements for imaging, and resulted in
a non-traditional approach of tightly integrating the imaging
and lighting system. Early in the system design process, it
was concluded that conventional full-frame rate imaging was
not necessary for many of Nereus’ tasks, and that this could
exploited to reduce imaging and lighting power requirements
.Minimization of frame rates is particularly valuable in ROV
mode, when light output is important to producing high quality
color imagery. Such a system requires non-traditional lighting
sources capable of tailoring their output to the needs of
the imaging cameras. Newly available high brightness LEDs
offered a means to strobe at high rates, as well as offering
other benefits such as pressure tolance and precise control
over lighting patterns. For ROV mode imaging, a full-motioncapable color imager based upon a machine vision camera
used in previous deep sea work [16] has been employed.
The camera is capable of capturing a high resultion color
image whenever it is triggered, at rates up to 29.97 frames
per second. The camera trigger causes LED strobe discharge
with a selectable pulse length thus allowing effective intensity
to be controlled. The resulting system thus uses power to make
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light of the required intensity only when required. A topside
computer collects and buffers the imagery, so that the viewer
only sees illuminated frames of imagery.
Triggering at these varying frame rates and durations required the development of a custom LED driver controller
board.. The controller is told via software the expected frame
rate. This rate is diven by the needs of the pilot and depends
entirely upon the need for visual update about vehicle activities. In another attempt to save power, only areas viewed
by the cameras are illuminated. A 16-LED assembly or puck
was designed. The lighting array is composed of multiple,
individually controlled pucks, each capable of illuminating
a 30 degree frustum. By selecting the individual pucks to
be triggered, we can direct the light only to areas requiring
illumination. The LED controller is directed via software to
trigger those individual pucks. The pucks were the result of a
custom design effort. They contain 16 LEDs in a reflector
carefully designed to create an even lighting pattern. The
pucks are fully pressure compensated, and have been tested
to 11000 meters. Pressure compensation of the assemblies
results in dramatic weight savings over traditional pressure
resistant lighting enclosures. The Nereus design includes 18
strobing LED pucks, arranged in an array on the nose of one
of the hulls. Two other pucks, used in a continuous lighting
mode (not strobed) are also included in the design as backups
in the event of controller failure. In addition to the custom
motion camera, two conventional subsea video cameras in
11000 meter housings support viewing of the work package
and other utility tasks. Figure 9 shows a preliminary mounting
of several of the pucks during the Nereus sea trials. All of
the imagery, including the conventional video camera data, is
brought to the surface for viewing and recording over gigabit
Ethernet channels. Nereus AUV imaging is more conventional,
utilizing an array of eight of the samedesign pucks, and the
same LED controller, supporting cyan LEDs in support of a
grey scale
VIII. P OWER AND P ROPULSION
Nereus is powered by an 18 kWh rechargeable lithiumion battery system configured to provide a 50 volt bus.
Building upon lithium-ion experience with the ABE and
REMUS AUVs, WHOI designed a modular building block
of 12 cylindrical cells. The process included a comprehensive
safety analysis followed by independent laboratory testing for
conformance to United Nations shipping regulations. The 12cell packs are assembled into two identical chassis on Nereus
and combined electrically onto a common bus providing up to
3 kW continuous for all systems.
Power is distributed to low power devices from both sides
of the vehicle in order to minimize cross-vehicle wiring. Both
port and starboard chassis contain low power switches that
feed individual power conversion units. These units provide
electrical isolation, allowing for individual ground fault detection and minimization of ground loops, and are roughly
sized according the loads for maximum efficiency. High power
devices are few in number and are all switched from a single
chassis. These include propulsion, control surface, and lighting
devices, each consuming up to 1 kW. High power switches are
built around solid-state relays, controlled by a custom interface
circuit board that includes soft start as well as voltage, current,
and temperature sensors.
The propulsion system is the largest load on the power bus,
and therefore has the greatest impact on mission duration.
Hydrodynamic modeling indicated the drag of the two-hull
AUV geometry would be 230 N at the target speed of
1.5 m/s. The corresponding electrical energy fell within the
overall capacity of the power system. AUV mission profile
simulations, which estimated the energy used by the rest of
Nereus systems as well as propulsion, yielded reasonable AUV
mission lengths.
The design of the electric thrusters started with the propeller.
The technique of Froude scaling allowed measured efficiency
of different pitch and diameter propellers to be compared.
Using this technique, a two bladed carbon fiber prop with 0.75
m diameter and 0.56 m pitch was specified and fabricated. A
brushless permanent magnet electric motor and 7:1 planetary
gearbox were selected to maximize transmission efficiency at
200 RPM. Using a gearbox instead of a larger direct drive
motor yielded a 75% smaller and lighter thruster package.
As a result of design choices favoring AUV propulsion,
ROV maneuvering is not fully optimized. Vertical thrust is
limited to 350 N, so buoyancy must be monitored carefully,
and yaw rate is limited by the long swing of the aft thrusters.
A lateral thruster is added with the work package, improving
worksite maneuverability.
The November 2007 sea trials demonstrated that Nereus
in ROV mode is a power efficient yet capable sampling
platform. During a dive on November 25, 2007, the vehicle
was submerged for 6 hours and 50 min, with 3 hours, 21
min on bottom at 2270 m depth. Successful manipulation
exercises were performed on glass bottles, a can, a large piece
of metal debris, and several sediment push cores, all while
covering 2200 m in horizontal distance. Including predive and
recovery, 100.9 Ah battery charge was used out of a total 369
Ah available, approximately 27% of capacity.
IX. NAVIGATION
Nereus’s extreme operating depth presents challenges for
navigation sensing and estimation. Externally mounted navigation sensors such as the pressure depth sensor, Doppler
sonar, and LBL transducers must be capable of withstanding
the pressure at 11,000m. In 11,000m operations, the long
acoustic paths of conventional long baseline (LBL) acoustic
navigation [17] gives rise to problems of signal attenuation,
decreased accuracy, and limited update rates. These challenges
motivate the need to combine LBL navigation with Doppler
navigation [20], [34] and inertial navigation [4], [21], [22] in
order to obtain the precise, geodetically referenced navigation
necessary for closed-loop control and benthic science.
The Nereus navigation sensor suite includes a Paroscientific
pressure depth sensor, a RDI 300kHz Doppler sonar, an
Ixsea Phins IMU, a LBL transceiver, an WHOI Micro-Modem
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Fig. 10. Nereus Dive NER005: Screen shot of the DVLNav program, [20],
during dive NER005 at 2,257 m depth during closed-loop trackline navigation
and control testing.
[12], and a Microstrain gyro-stabilized attitude and magnetic
heading sensor. The Doppler sonar provides 3-axis bottomlock vehicle velocity with respect to the sea floor at over 200 m
altitude, 3-axis water-lock velocity of the vehicle with respect
to the water, and 3-axis water column velocity profiles. The
IMU contains a 3-axis North-seeking fiber-optic gyrocompass
providing attitude and heading at 0.01◦ accuracy.
Navigation sensor data is received by Nereus’s control computer. The navigation process NavEst computes vehicle state
estimates. NavEst is navigation software developed by WHOI
and JHU for use on deep submergence vehicles. Presently
employed on the Sentry AUV and Nereus, NavEst is a multithreaded Linux program that supports multiple simultaneous
navigation algorithms. Available navigation algorithms include
the Doppler navigation algorithm employed by DVLNav [20]
and the LBL algorithm extensively used by the ABE AUV
[36]. Single beacon one-way travel time algorithms have also
been implemented [9] .
During the November 2007 engineering trials, Nereus realtime navigation employed a pressure depth sensor, a Phins
IMU, and Doppler sonar. Fixed seafloor transponders were
not deployed, precluding the use of LBL navigation. The
sensors and NavEst software provided vehicle state estimates
that enabled closed-loop trajectory tracking control in ROV
mode and in AUV mode. In ROV mode, navigation data is
broadcast via Ethernet to computers topside and visualized
using the DVLNav software package [20]. In AUV mode,
DVLNav displayed basic vehicle navigation and engineering
state information provided by the acoustic modem. Figure 10
shows a screen shot of the DVLNav program, [20], during
ROV mode dive NER005 at 2,257 m depth during closed-loop
trackline navigation and control testing.
X. C ONTROLS AND DYNAMICS IN AUV M ODE
When configured as an AUV Nereus is designed to be
efficient in forward flight at speeds up to 2 m/s. Unlike
most AUVs, Nereus can stop, hover, and reverse direction
Fig. 11. Nereus’s depth and pitch control modes [18]. Lift-dominated low
angle of attack flight modes will enable efficient operation at high speeds.
Thruster-dominated high-angle of attack hover modes will produce the climb
angles necessary to negotiate steep terrain at low speeds. Foils-fixed hover
and the two flight modes were demonstrated successfully in the recent sea
trials.
if necessary to negotiate steep terrain. The ABE vehicle has
demonstrated the value of this latter capability for seafloor
scientific surveys [35]. However, unlike AUVs with static
thruster configurations (ABE, SeaBED) or more typical finactuated AUVs, Nereus’s depth control strategy changes as a
function of speed. This increased complexity comes with the
substantial benefits of low cruising drag and alignment of the
thrusters with the predominant flow direction except at very
low speeds. Dynamics and controls experiments conducted
during sea trials in fall 2007 were focused on demonstrating
vehicle control in the longitudinal plane (surge, heave, and
pitch) and identifying hydrodynamic parameters critical to
maximizing vehicle performance during survey and highefficiency homing descents.
A. Longitudinal Plane Control
Nereus’s unusual foil-mounted-actuator configuration creates a variety of potential strategies by which the vehicle can
control its depth. The efficiency and performance of each of
these is dominantly a function of forward speed, with higher
speeds favoring low-angle of attack motions to limit drag
losses. Figure 11 illustrates Nereus’s gain-scheduled strategy
for depth control. Extensive simulation studies [18] were
utilized to develop the following three depth controllers:
1) Foils-fixed hover: Hovering for low speed, high angle of
attack maneuvers.
2) Zero-pitch flight: Level flight for intermediate to high
speed survey at low to medium angles of attack.
3) Zero-w flight: Zero body-frame vertical velocity (zerow) pitching flight for efficient high speed climbs at low
angles of attack.
The three control modes were demonstrated successfully
during the sea trials and tuned for acceptable performance.
Automated switching between level flight and foils-fixed hover
was demonstrated during a high-speed terrain-following experiment at 5 m altitude, as depicted in Figure 12.
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B. Terrain-Following
The utility of Nereus as a imaging survey AUV is dependent
upon its ability to terrain-follow (maintain a constant altitude
above the seafloor). The terrain-following algorithm implemented on Nereus is modeled on the algorithm employed on
the successful ABE AUV [35], [36]. The algorithm attempts to
limit control action and provide robustness to noisy altimeter
data by maintaining the depth setpoint within a prespecified
envelope above the seafloor rather than servoing off altitude
directly.
In the 2007 sea trials Nereus successfully flew a 5 m altitude
photo-survey over modest terrain. The result (Figure 12) is
noteworthy in that despite the relatively smooth seafloor encountered, two outcroppings were sufficiently steep to require
higher climb angles than were expected to be possible in level
flight mode. The control program responded to these obstacles
by autonomously switching actuator allocation modes from
level flight to foils-fixed hover. These results were attained
without the benefit of feed-forward or integral action in the
depth controllers. These features have been implemented and
will be engaged in subsequent trials.
XI. M ISSION C ONTROLLER
This section reports the progress towards developing a mission controller for the high level control of the Nereus HROV.
The mission controller performs the job of the pilot in the
absence of a telemetry system for Nereus. In AUV mode, the
mission controller supports fully-autonomous survey missions.
In ROV mode, the mission controller permits normal pilotcontrolled teleoperation of the vehicle and in the event of the
loss of tether telemetry will assume control of the vehicle and
autonomously complete a preprogrammed mission.
The mission controller manages the mission by executing a
high-level mission plan, and respond to events. At present the
mission controller is capable of the following:
1) Condition Monitoring: continuous monitoring, detection
and response to vehicle subsystem status including vehicle depth and Doppler sonar status.
2) Telemetry Status Monitoring: detection and response to
status of fiber-optic Ethernet link.
3) New Mission Execution: the ability to dynamically load
and execute new mission scripts.
4) Simulation Mode: fast-time simulated missions.
5) Simple Command Primitives: simple primitives for control of speed, depth, altitude, and heading.
6) Trackline Following: trajectory control for survey operations.
7) Abort Mission: the ability to perform a controlled mission abort.
XII. ACOUSTIC T ELEMETRY
The acoustic telemetry system is designed to send data
acoustically between multiple vehicles including one or more
surface ships. The system employs Micro-Modems, [12], designed at Woods Hole Oceanographic Institution (WHOI) and,
Fig. 13.
Vehicle trackline overlaid on vehicle positions transmitted via
acoustic uplink.
in the case of the Nereus vehicle, EDO/Straza SP23 acoustic
transducers.
The Acomms program is a Linux daemon which runs on
the Nereus control computer. It manages the WHOI MicroModem, serving two main roles. First, the Acomms process is
designed to monitor and report on the status of the modem and
keep it properly configured through modem reboots. Second,
the Acomms process is designed to act as a transport layer
between the vehicle program and the modem for sending and
receiving acoustic data, providing basic format checking for
messages sent to the modem and exhaustively logging all
Acomms related data.
For the recent sea trials, Nereus was equipped with one
Straza transducer mounted on the forward starboard hull of
the vehicle facing upwards. A second transducer was deployed
from of the stern of the ship (the R/V Kilo Moana) facing
downwards. The ship transducer was lowered several meters
underwater and held in place using a bridle that prevented
side-to-side motion.
During vehicle-ship communication, the vehicle is designated as the master, initiating all acoustic communication.
Messages are sent out in a time division multiple access
(TDMA) cycle that is configured at run time. The slave
acomms process on the ship responds to acoustic queries as
necessary and can also send acoustic abort signals. During
Nereus trials the following acoustic messages were employed:
• An Abort message indicates that the vehicle should run
its abort script.
• A Ranging Ping interrogates another modem and returns
a travel time between the sender and the receiver.
• A Cycle Init message requests data and designates which
modem is to send and which is to receive.
• A Vehicle Status message consists of 32 bytes of data
containing vehicle status information (position/attitude,
velocity, and battery charge).
• An LBL Ping interrogates long baseline (LBL) acoustic
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Fig. 12. Terrain-follower performance versus distance travelled during a mock photo-survey at 5 m altitude. Times when the terrain-follower was actively
controlling depth are indicated by the grayed areas. The algorithm autonomously modulated speed to keep the reference trajectory within a 1 m envelope
throughout the majority of the survey. This included autonomous switching into foils-fixed hover allocation from level flight allocation when the desired climb
angle exceeded that attainable in level flight. Mode switch times are indicated by heavy vertical bars (green: hover→flight; red: flight→hover).
beacons at up to four distinct frequencies and reports
travel times.
In the November 2007 sea trials we demonstrated the ability
to send and receive abort packets as well as respond to
data requests with a Vehicle Status message. This vehicle
information proved invaluable during AUV trials, allowing the
users to track the progress of the vehicle in real time during its
mission. Figure 13 shows an example of status (position) data
received acoustically at the ship in real time and the actual
vehicle track line.
In addition, the LBL functionality of the Acomms process
has been fully tested in an LBL transponder net subsequent
to this cruise, where we have demonstrated the ability of the
navigation program to parse Micro-Modem LBL messages and
calculate the resulting vehicle position.
XIII. C ONCLUSION
For the past 50 years, vehicle limitations have restricted
routine direct access to depths of 6,500 m or less. Only a few
deeper vehicles have ever been developed and successfully
deployed. The scientific community has established substantive imperative to investigate the deep ocean floor at depths
below 6500 m, yet a lack of practical technology prevents
routine access to the deepest ocean. This virtually unexplored
area of the ocean almost certainly offers the potential to make
important biological and geologic discoveries. Preliminary sea
trials with Nereus in November 2007 demonstrated basic
functionality of capabilities in navigation, control, mission
control, fiber optic tether, and acoustic telemetry necessary for
AUV mode autonomous survey missions and ROV mode sampling missions in a single vehicle package. This development
points to a way forward for both scientific and commercial
operations through the use of a unique combination of adapted
technologies. Future sea trials are planned to demonstrate the
vehicle’s operational capability for operations at depths to
11,000 m.
ACKNOWLEDGEMENTS
In March, 2004, WHOI engaged the noted ceramics experts
Dr. Jerry Stachiw and Dr. Joan Stachiw of Stachiw Asssociates
to assist with the development of ceramic pressure housings
and flotation for the vehicle. They led the effort to develop,
procure, and test the ceramic components on Nereus, and
provided invaluable technical support througout the program.
We regret that Jerry Stachiw passed away on April 25, 2007.
Our heartfelt thanks go to Joan for all of her and Jerry’s
contributions to the Nereus project
The Nereus project has benefited from external review
from an independent advisory committee whose membership
includes the following: Patricia Fryer (University of Hawaii),
Lawrence Lawver (University of Texas at Austin), Chuck
Fisher (Pennsylvania State University), Deborah Kelley (University of Washington), Keir Becker (University of Miami),
and James McFarlane (International Submarine Engineering
Ltd, British Columbia, Canada).
We are grateful to Captain Rick Meyer, the Officers, and
Crew of the R.V. Kilo Moana who capably supported the
Nereus team during the November, 2007 sea trials.
We are grateful to the entire Nereus project team, which
includes, in alphabetical order: John Bailey, Lisa Borden,
Andrew D. Bowen, Albert Bradley, James Buescher, Megan
Carroll, Tito Collasius Jr., Thomas Crook, Al Duester, Daniel
Duffany, Paul Dugas, Geoff Ekblaw, Robert Elder, A. B.
Explorer, Norman Farr, Judy Fenwick, Barbara Fletcher,
Daniel Fornari, Phillip Forte, Robert Fuhrmann, Alan Gardner, Chris German, Daniel Gomez-Ibanez, Christopher Griner,
Mark Grosenbaugh, Edward Hall, Matthew Heintz, Jeff Hood,
Jonathan Howland, Michael V. Jakuba, Craig Johnson, Richard
Johnson, James Kinsey, Tom Lanagan, Chris Lumping, Robert
McCabe, Glenn McDonald, Casey R. Machado, Stephen Martin, Carlos A. Medeiros, Cathy Offinger, Don Peters, Hugh
Pompano, Eric Schmitt, Stephen Schmitt, Will Sellers, Tim
Shank, Hanu Singh, Jerry Stachiw, Amy Stetten, Ann Stone,
Benjamin Tradd, Chris Taylor, Jim Varnum, Karlen Wannop,
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Sarah Webster, Louis L. Whitcomb, Lidia Williams, Dana R.
Yoerger, and Chris Young.
We gratefully acknowledge the support of the National
Science Foundation, the U.S. Navy Office of Naval Research,
the National Oceanic and Atmospheric Administration, the
Woods Hole Oceanographic Institution, and the Russell Family
Foundation.
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